It often is desirable to know the particle size distribution of particles dispersed in a liquid medium of an industrial process, e.g. a process plant pipe in which the medium is flowing or a chemical reactor. For these reasons, among others, various methods have been used in laboratories on samples from such processes to determine the characteristics of particles in such liquid medium.
One such characteristic is the particle size distribution (PSD). See, for example, U.S. Pat. Nos. 4,706,509, 5,121,629, and 5,569,844. U.S. Pat. No. 4,706,509 describes a method for ultrasonically measuring solids concentration and particle size distribution in a dispersion. Ultrasonic waves at a variety of frequencies are directed into the dispersion, and the attenuation at these frequencies is measured. A dimensional spectrum (across the range of particle dimensions) is divided into dimensional intervals, and a system of linear equations is developed to represent the concentration of particles in each dimension interval. The system of equations is then solved to determine the PSD.
The process described in U.S. Pat. No. 5,569,844 involves measuring the attenuation of both ultrasonic waves and electromagnetic radiation to determine particle size distribution. Specifically, ultrasonic velocity and ultrasonic attenuation are combined with the density, as determined from the electromagnetic radiation attenuation, to calculate the PSD. In addition to the problems inherent in generating x-rays or gamma rays, however, particle sizes of about 10 to 15 μm appear to be the lower limit for the process.
In U.S. Pat. No. 5,121,629, ultrasonic waves at a variety of selected frequencies are passed through a dispersion, and the attenuation at each frequency is measured to derive a measured attenuation spectrum over those frequencies. Separately, based on a theoretical model, a set of attenuation spectra are calculated for a variety of PSDs, and the calculated spectra are then compared to the measured spectrum to formulate a preliminary approximation of the PSD of the dispersion. Further calculations must be performed, starting from this approximation, to more accurately determine the PSD.
U.S. Pat. No. 6,119,510 describes an improved process for determining the characteristics of dispersed particles. The term particles is used to include solids, liquids, or gases dispersed in a continuous medium. Waves (acoustic or light) are directed into a dispersion, and the attenuation of the waves for particular frequencies is measured to provide an attenuation spectrum. The measured attenuation spectrum is then compared to a set of theory-based calculated attenuation spectra to determine the particle size distribution corresponding to the measured attenuation spectrum. Unlike previous processes, the particle size distribution is capable of being accurately determined by a single inversion algorithm. Inversion techniques involve taking a set of known particle size distributions, determining the attenuation spectrum that each PSD would theoretically produce, and comparing a set of such theory-based spectra to the actual, measured spectrum to find the actual PSD.
Acoustic attenuation techniques for characterizing particles in dispersions involve the interaction of applied sound waves with the dispersed particles. As a sound wave travels through a dispersion, the wave loses acoustic energy by various scattering mechanisms. Measurement of the attenuation at different frequencies of the sound wave leads to an acoustic attenuation spectrum. Models by Epstein and Carhart (Acoust. Soc. Am. 25, 553 (1953)), and by Allegra and Hawley (Acoust. Soc. Am. 51, 1545 (1972)) make it possible to predict the attenuation spectrum for particles of a given size distribution and concentration. The models require knowledge of several physical properties of the particles and the liquid medium, including density, thermal expansion coefficient, thermal conductivity, heat capacity, viscosity, and shear rigidity. It is then possible to construct a 3-D matrix that relates attenuation, frequency, and particle size.
Acoustic attenuation in a particle dispersion can be measured by use of an apparatus such as the Ultrasizer™, made by Malvern Instruments, Ltd., Worcestershire, United Kingdom (the assignee of U.S. Pat. No. 5,121,629, discussed above). A schematic of the chamber 110 of such an apparatus is illustrated in FIG. 7 of U.S. Pat. No. 6,604,408. There, two pairs of broadband transducers 112, 114, 116, 118, are in contact with a sample located in a tank 120, typically formed of stainless steel. Typically, one pair of transducers 112, 114 covers a lower frequency range, e.g., 1 to 20 MHz, and the other pair 116, 118 a higher range, e.g., 15 to 200 MHz. Generally, the transmitting transducers 112, 116 are capable of being moved to different positions in the chamber, but the receiving transducers 114, 118 are fixed. The spacing between the transmitting transducers 112, 116 and the receiving transducers 114, 118 is controlled by a stepper motor.
U.S. Pat. No. 5,121,629, discloses a through-transmission device useful for industrial on-line measurement and control of slurries that uses at least one pair of acoustic transducers wherein one of the pair is moved by a stepper motor in order to perform attenuation measurements at various acoustic path lengths.
Problems with the prior art devices, such as apparatus described above, include wear of the seal due to the translation movement of the receiving transducers. That wear causes misalignment of the receiving transducer with the emitting transducer causing major degradation of results due to minute levels of misalignment. The transducers move up to about four inches and, thus, considerable wobble can be experienced due to movement on the o-ring seal. The wear of the seals also can result in leaking of the sample, which can result in a change of the volume sample, i.e., the level of the sample in the sample cell changes. Further, the in/out movement of the variable position transducers in the sample cell and resultant leakage can cause significant changes in the sample level in the cell. The change in the volume/level of the sample can result in the introduction of sound waves reflected from the liquid surface or a change in the reflection pattern, thereby further degrading the results. Besides sample loss, leaks also can cause electrical damage to the device and sample change. For example, the percent of solids in the sample can increase if the medium is leaked. Also, the shear applied on samples by friction between a moving transducer and an o-ring can cause the particles to aggregate (to form clusters). This particle aggregation can shift the measured particle size to a larger value, thereby rendering the data inaccurate.
Constant wear and tear of the o-ring seal and transducer causes progressive loss of alignment that leads to progressive degradation of data. Thus, frequent replacement of the o-ring seals is required along with tedious alignment of the transducers after each replacement.
In order to produce true, accurate, high-resolution, broad-particle-size range PSD data, Acoustic Attenuation Spectroscopy measurements are made over a wide acoustic frequency range of at least 4-20 MHz and preferably 4-80 MHz or higher. Particles attenuate sound more efficiently at acoustic wavelengths close to their particle size, i.e., larger particles attenuate sound more efficiently at low frequencies while smaller particles do so at higher frequencies of the sound wave.
In order to achieve such broadband (true) spectra, attenuation measurements typically are made at a minimum of two acoustic path lengths. Using multiple path lengths enables measurements over broader frequency ranges due to the fact that attenuation increases monotonically with frequency, i.e., optimal signal-to-noise measurements can be performed as follows: higher-frequency data (where attenuation is higher) can be collected at shorter paths while low-frequency attenuation data can be measured at longer paths.
Measurements at multiple path lengths also enable the determination of the acoustic fixed loss at the various sensor interfaces such as within the ultrasonic transducer delay rod, and at this rod's liquid interface, and acoustic reflectors, if present (see Reed, R. W., DosRamos, J. G., and Oja, T., Review Quant. Nondestr. Eval, 21, Thompson, D. O. and Chimenti, D. E., Ed., pp. 1494-1501 (2001)).
U.S. Pat. No. 6,604,408, discloses a device that uses an acoustic reflector introduced through the top of a sample cell. The reflector is moved within the sample cell to position it at various path lengths to measure the returning acoustic echoes from two transducers. This device eliminates the need to move acoustic transducers through walls of the sample cell.
The use of moving parts is undesirable for continuous process on-line operation, and the relatively long time required for the acquisition of the attenuation-spectrum data due to the need to re-position the transducers or reflectors hinders or prevents real-time data generation.
Although current apparatus for characterizing dispersed particles, e.g., determining PSD, are adequate, improvements to devices that can determine PSD on-line, and particularly in real time, in industrial processes are desired.